On the eve of D-Day, Nazi agent Paul Fidrmuc – codename Ostro – was the man who knew too much. Days before the Normandy landings, the Lisbon-based spy got wind of the final details of Operation Overlord and sent an urgent message to his Berlin handlers. The Allies were not planning to land in Calais, as the Nazis thought and where they had massed 200,000 soldiers. Instead, he wrote, “the preferred plan is around La Manche”.

“It involves an air-and-sea operation against the channel islands, landing east and west of the La Manche Departments, possibly at Isigny (11’ east of Carentan on the eastern site of Cherbourg).”

Had his message been heeded, the Allied surprise landing in Normandy might have developed very differently and the decisive turning point in their second World War fortunes could have ended in disaster.

The man who knew too much was born in 1898 in Lundenburg, 70km north of Vienna, now Breclav in the Czech Republic. A reserve officer in the first World War, he began studying in Vienna in 1920 but broke off his studies, moved to Lübeck, began working in the metal export business and married a Danish woman.

Interrogated

By the mid 1920s he tried his hand at journalism and was already in the sights of both German and British secret services. An early British report rates Fidrmuc as highly intelligent, able to speak six languages besides his native German: English, Italian, French, Danish, Spanish and Portuguese.

In 1935 he was arrested by the Gestapo and interrogated in Berlin suspected of being a double agent. In later life he admitted as much, claiming to have acquired in advance plans for the annexation of the Sudetenland and Austria.

Fidrmuc joined the Nazi Party in May 1939 and, a year later, moved to neutral Lisbon. In this hotbed of arms dealers, deserters, spies and refugees desperate to get across the Atlantic, he gathered US and British intelligence for the Nazis while London fed him fake information in the hope of discrediting him. Unknown to them, Fidrmuc already enjoyed a reputation among his handlers as a fabulist and Nazi leaders dismissed his D-Day report as more of the same.

“Of the many messages we received,” said Adolf Hitler on June 6th, “there was one that predicted precisely the landing site, with precise day and time. It was this that made me sure it couldn’t be the actual invasion.”

Fidrmuc, who served as a source of inspiration for Graham Greene’s Our Man in Havana, remained a true believer in “National Socialist revolution” which, he wrote later, “couldn’t have succeeded in the early years without draconian means”. From 1950 until his death eight years later he worked as Spanish correspondent with Der Spiegel magazine.